1. Field of the Invention
This invention relates to the manufacture of printed circuit boards having a copper foil layer laminated to a dielectric substrate, and more particularly to a treatment to increase the adhesion of the copper foil layer to the dielectric substrate.
2. Description of the Related Art
Copper and copper base alloy foils are widely used in the printed circuit board industry. The foil is produced to a thickness of under 0.008 inches and more generally to a thickness in the range of from about 0.0002 inches (known in the art as ⅛ ounce foil). The foil is produced by one of two means. “Wrought” foil is produced by mechanically reducing the thickness of a copper or copper alloy strip by a process such as rolling. “Electrodeposited” foil is produced by electrolytically depositing copper ions on a rotating cathode drum and then peeling the deposited strip from the cathode.
The copper foil is bonded to a dielectric substrate forming a printed circuit board using a lamination process. The dielectric substrate is typically a fiberglass reinforced epoxy such as FR-4 (a fire retardant epoxy) or a polyimide such as Kapton® manufactured by DuPont. The lamination process includes bonding the copper foil layer to the dielectric substrate through the use of heat and pressure. A pressure of about 300 pounds per square inch (psi), at a temperature at about 175° C. for a time of up to 30 minutes will provide suitable adhesion between the layers.
To maximize adhesion, it is often desirable to roughen the surface of the foil that contacts the dielectric substrate prior to bonding. While there are a variety of techniques available to roughen or treat the foil, one exemplary technique involves the formation of a plurality of copper or copper oxide dendrites on the foil surface. U.S. Pat. Nos. 4,468,293 and 4,515,671, both to Polan et al. disclose this treatment. This treatment is referred as the CopperBond® treatment. CopperBond is a trademark of Olin Corporation of Norwalk, Conn. Another electrolytic surface roughening treatment is the deposition of copper/nickel nodules onto the surface of the foil that contacts the dielectric substrate, as disclosed in U.S. Pat. No. 5,800,930 to Chen et al. In some instances, at least one side of the foil, particularly the roughened side bearing the dendrites, may have an electrodeposited coating of zinc or brass applied thereto. This coating has been found to enhance the bond strength of the foil with the dielectric substrate.
While the use of roughened surfaces on the copper foil is effective to promote adhesion with the dielectric substrate, the degree of surface roughening is often restricted by the electrical performance requirements of the copper foil for high frequency applications. Problematically, decreasing the surface roughness to meet these electrical performance requirements compromises the adhesion (peel strength) between the copper foil and the dielectric substrate.
Another problem facing printed circuit board manufacturers using either electrolytic or wrought copper foils is the relative reactivity of the copper. As a result, copper readily stains and tarnishes. The stains and tarnish are aesthetically unpleasant and may be a source of problems during the manufacture of the printed circuit board. For example, staining of copper foil prior to lamination can affect both the bond strength between the foil and the dielectric substrate and the etching characteristics of the resultant laminate. The tarnish resistance of a copper foil may be enhanced by applying a thin (can be on the atomic scale) coating that contains co-deposited ions of zinc and chromium. This treatment, referred to as the P2 treatment, is disclosed in U.S. Pat. No. 5,022,968 to Lin et al.
The U.S. Pat. Nos. 4,515,671; 5,800,930 and 5,022,968 patents are incorporated by reference in their entireties herein.